We have been studying the role of Ig receptor expression on B cell development. Initial studies in X-linked immune deficient (Xid) mice transgenic for anti-phosphocholine (PC) specific IgH/Lchain genes revealed that B cells expressing anti-PC specific sIg-receptors failed to populate primary and secondary lymphoid organs. Introduction of the bcl-2 transgene into the Xid anti-PC specific -sIgH/L mice rescued anti-PC specific B cells demonstrating that bcl-2 could alter B cell receptor-mediated selection at late stages in B cell development. Additionally these studies suggested the possibility that these anti-PC sIg receptors were autoreactive. The autoreactive nature of anti-PC -IgH/L expressing B cells was confirmed by introducing the anti-PC IgH/L transgenes into Rag-2 knockout (Rag-/-) mice. These anti-PC receptor expressing B cells were developmentally arrested in the bone marrow. This is in sharp contrast to the large numbers of anti-PC receptor expressing B cells that are observed in the peripheral lymphoid organs of normal mice (Rag+/+). As observed in the Xid experiments, over expression of bcl-2 allowed anti-PC-IgH/L expressing B cells to populate the peripheral lymphoid organs in Rag-/- mice. However, these anti-PC-IgH/L expressing B cells did not mature into CD23+ B cells. Further experiments demonstrated that coexpression of additional IgL chain genes or IgH chain genes could rescue the anti-PC-sIgH/L chain expressing B cells from developmental arrest in Rag-/- mice and allow them to populate the peripheral lymphoid organs. Co-expression of multiple Ig receptors on a single B cell violates the principle of allelic exclusion. However, there may be a biological reason for compromising allelic exclusion i.e. anti-PC specific antibodies protectagainst infection by Streptococcus pneumoniae. Compromising allelic exclusion allows the host to retain the necessary tools to protect itself from this potentially lethal pathogen. One possible mechanism by which coexpression could rescue autoreactive anti-PC sIgH/L chain expressing B cells is through receptor dilution. Model systems are being developed to examine the generality of the coexpression observations in normal mice and to delineate the repertoire of candidate IgL chain genes which are capable of rescuing autoreactive anti-PC specific receptor expressing B cells by coexpression. What these data suggest is that autoreactive antibodies that nonetheless have the capacity to protect the host from a life-threatening pathogen find a way to avoid tolerance induction and/or clonal deletion during development. B cells expressing these protective but autoreactive antibodies survive by violating the principle of allelic exclusion. Instead of each clone of B cells expressing only one type of immunoglobulin molecule, B cellsexpressing anti-PC antibodies (which also recognize platelet activating factor) reduce the number of antigen binding sites on the cell surface by diluting the receptor binding. This dilutionis accomplished by rearranging a second light chain and making antibodies that contain the autoreactive/protective specificity and antibodies that have an unrelated specificity in the samecell. This receptor dilution lowers the threshold for antigen binding below the level that would get the cell killed and allows the animal to retain a potentially life-saving antibody. We analyzed a large number of H + L receptor combinations in Rag transgenic mice to try and determine how extensive formation of autoreactive B cells is for a given H chain. Individual mouse strains expressing three different variant forms of a single H chain were crossed to individual strains of Rag mice which expressed one of five different L chains. B cell development was analyzed in the spleen and bone marrow of the 15 combinations generated. Only 5 of the 15 H + L combinations were able to give rise to significant numbers of peripheral B cells. In most of the mice, B development was arrested in the bone marrow suggesting that the majority of H + L receptor combinations gave rise to autoreactive B cells. Surprisingly, the T15H + kappa 22 L chain combination, which is know to bind to several self antigens, produced B cells even in these Rag deficient mice. This indicates that some autoreactive B cells escape clonal deletion even when they can not modify their receptors by expressing more than one L chain or by rearranging a new L chain.